A common means by which to locate a device is to determine the amount of time required for signals transmitted from multiple sources at known locations to reach the device. One system that provides signals from a plurality of transmitters at known locations is the well-known Global Positioning Satellite (GPS) system. Satellites in the GPS system are placed in precise orbits according to a GPS master plan. The locations of the GPS satellites can be identified by a number of different sets of information, some more accurate than others.
GPS satellites transmit a set of information, referred to as “Almanac”, which includes less accurate information regarding the location of the satellites in a “constellation”. Ground stations continuously monitor the GPS satellites to observe their variations in orbit. Once the precise locations of the satellites have been determined, the information is relayed back to the satellites. The satellites then transmit another set of information, referred to as “Ephemeris”, which includes a higher accuracy version of the satellite orbits. Each satellite transmits the Almanac information for all satellites in the GPS system but the Ephemeris information only for itself.
The Almanac and Ephemeris are valid for a limited amount of time. The Almanac information is considered to be accurate to approximately three kilometers for approximately one week from the time the Almanac is transmitted. The Ephemeris provides information regarding the satellite orbit with an accuracy of approximately one meter for approximately two hours. The error in both the Almanac and Ephemeris grows as the information ages. Accordingly, the location of the satellites based on this information is less and less accurate as the Almanac and Ephemeris age, unless updated information is received at appropriate time intervals.
A GPS receiver can receive and/or store an Almanac that indicates where each of a number of satellites is located in the sky at a given moment. A more accurate location of a GPS satellite may be determined based on the Ephemeris and the time of day at which this information is available.
Newer generation wireless devices (e.g., cellular telephones) are able to perform position determination based on GPS satellites and/or terrestrial base stations. For position determination based on satellite transmissions, the estimated location of a device may not be accurate if information regarding accurate locations of the satellites is not available. Accurate satellite locations may be attained by receiving updates (continually or as necessary) from the satellites themselves or from an alternative source. Such alternative source may be a base station or a position determining equipment (PDE) in a wireless communication system, each of which may be equipped with a GPS receiver capable of attaining the required information from the GPS satellites.
The messaging of position location data between a base station and a terminal is standardized in TIA/EIA/IS-801, entitled “Position Determination Service Standard for Dual Mode Spread Spectrum Systems,” Nov. 1, 1999 (referred to herein as the IS-801 standard), which is incorporated herein by reference. The IS-801 standard defines a communication model whereby IS-801 messages are transmitted point-to-point between the base station and terminal. A larger portion of the system resources may thus be required to transmit position location data from the base station to a number of terminals. Since system resources are typically limited for most wireless systems, these IS-801 messages may consume a significant portion of the available system resources.
There is therefore a need in the art for techniques to efficiently provide position location data (e.g., GPS Almanac and Ephemeris) to a number of terminals in a wireless communication system. This need is particularly acute when position determination is based on transmitters (e.g., GPS satellites) that move over time and accurate locations of such transmitters are known only upon receiving updates from the transmitters or an alternative source such as a base station.